DO NOT MERGE

Use a static variable and methods for the current WebView.

sJavaBridge may not exist by the time onWindowFocusChanged is called.
To avoid an NPE, just static methods that modify a global field.
JWebCoreJavaBridge is a global object anyway so using a static field
will be fine.  To avoid any garbage collection issues, store the
WebView in a WeakReference.

Bug: 2908023
Change-Id: I05e9261f2c3d13c10c73c9b34f3aeea1d12a08a6